The properties of antibodies to steroid-protein conjugates were investigated many years ago by Lieberman and his coworkers (1). Recently antibodies to an estradiol-protein conjugate have been used for assay of estradiol in human plasma (2). This report describes a method, using antibodies to an aldosterone-protein conjugate, to measure aldosterone in extracts of plasma purified by one column and one paper chromatography. MATERIALS AND REAGENTS 1,2-3H-aldosterone (New England Nuclear Corp., 29.5 c/mM) was purified every two weeks by paper chromatography in the system benzene:methanol:water 100:50:25. The purity of the labelled steroid was also examined once by acetylation and chromatography of the diacetate. Aldosterone was used as obtained from Mann Research Laboratories. The source and purification of solvents and the preparation of columns were as previously described (3). Silica gel (Merck, no. 7734) purified as previously described for Al2O3 (3). The source and method for washing chromatography paper and precoated thin layer chromatography plates and the composition of the scintillation fluid were as described earlier (4).